A firearm would do you little good because eventually you are going to run out of ammo.

This is a hypothetical situation and per the OP, YOU HAVE UNLIMITED MAGS AND AMMO. It's fictitious. Not real. Not realistic. Not even possible. But for the sake of argument, you WILL NOT RUN OUT OF AMMO.

I am in agreement with the 870. It will do all jobs any other single firearm will do. Other firearms "specialize" in performance and can surpass the 870 in individual tasks, but the shotgun will put food on the table and keep the boogie men away from the table.